"The strength and microstructure of a glass-ceramic material of the composition 53 percent SiO₂, 19 percent Al₂O₃, 15 percent MgO, and 13 percent Li₂O were studied. Cylindrical specimens of glass were formed and heat-treated to six different temperature levels for various time periods. The method of strength testing used was a diametral compression loading technique. It was found that the strength of this material increased initially with heat-treatment but then decreased. The strength characteristics of the completely crystallized glass ceramic material were found to be dependent on the initial nucleation period. Some factors which influenced anomalous crystal growth are described. The relationship of stress and nucleation in this study is discussed. Mechanisms of nucleation and crystallization were observed by transmitted and reflected light microscopy. The phases at various time periods were determined by X-ray diffraction, and the changes in softening characteristics were recorded by means of thermal dilatometer tests"--Abstract, page ii.